This invention relates to a circuit arrangement for testing a circuit point, which in a non-disturbed condition floats freely between a first and a second pole of an operating voltage in a given mode of operation. The circuit point is tested for a disturbed condition in which there is a leakage resistance relative to at least one pole of the operating voltage. The circuit arrangement includes at least one test circuit comprising a current source coupled to the circuit point to apply at least a given test current to this point, and having an evaluation circuit comprising at least one comparator having one input coupled to the circuit point to supply a first output signal when a current flows through the leakage resistance.
Such a circuit arrangement is known from EP 0,486,114 A2 which is hereby incorporated by reference. The current source in the test circuit is constructed as a current mirror which is connected to one pole of a high voltage source and which converts a current, which flows through the series arrangement of an object under test and two resistors and whose value is determined by the values of these resistors and the voltage of the voltage source, into an output current, which flows through a series arrangement of two further resistors. If a leakage current flows to one pole of the voltage source via the object under test, different currents will flow in the two resistor branches, which currents produce a corresponding difference voltage between the nodes of the series-connected resistors, which difference voltage is measured by means of one or more comparators. Thus, in this known circuit a current is applied to the object under test, which current is substantially larger than the maximum leakage current flowing through the leakage resistance to be monitored and which is determined by resistors disposed between the circuit point to be tested and one pole of the operating voltage, the part of the current mirror which supplies a current feeding a branch which is not connected to the object under test.
The circuit point to be tested is often situated within an extensive circuit, particularly an integrated circuit, or it forms an external output of such a circuit. Moreover, leakage-resistance testing should be effected not only on a single occasion, for example, during fabrication, but a continually repeated testing should be possible in normal operation. It is then not permissible to apply to the circuit point to be tested a current which is substantially larger than a current through an admissible leakage resistance, but the impedance at the circuit point should be as high as possible under all operating conditions, i.e. both test and normal operation.